High voltage powerlines carrying voltages in the hundreds of kilowatt range have been constructed to extend from spaced towers high above terrain which is often practically inaccessible to land vehicles. In recent years, a number of methods and devices have been developed to facilitate the repair of remote high voltage powerlines which are accessed by helicopter.
Helicopter transported landing platforms and work scaffolds which can be attached to the cross arm of a utility pole have been developed to permit a lineman to disembark from a helicopter and descend vertically along the utility pole to the powerlines supported thereby. Such devices are heavy and bulky, and are often difficult to maneuver. More importantly, however, is the fact that these devices position a lineman in close proximity to a grounded utility pole, so that the powerlines must be deenergized before repair can occur. Otherwise it is dangerous for a lineman to remain in the electrically charged space between energized high voltage conductors and a grounded tower or utility pole. A helicopter-mountable landing platform and work scaffold adapted to be mounted by a utility pole is illustrated in U.S. Pat. No. 3,863,736 to McWilliams.
In order to avoid an extensive power outage which might occur when a high voltage powerline is deenergized for repair, it is desirable to provide live maintenance on energized high voltage conductors. To accomplish this, linemen first bond to the energized transmission line to become energized to the same potential as that of the line. This permits the lineman to work safely on the energized line so long as he maintains sufficient spacing from grounded structures, such as support towers, to prevent the risk of flashover. To facilitate live maintenance on energized high voltage powerlines, various lineman support structures have been developed which are suspended from a helicopter over a powerline pair remote from the support towers. These support structures are formed either partially or totally from electrically conductive material and include skids or rollers which engage a pair of parallel lines. Such lineman support structures are shown by U.S. Pat. Nos. 4,478,312 to Kurtgis and U.S. Pat. No. 5,494,132 to Hansen. Not only are these structures bulky and somewhat cumbersome, but it is difficult to position the conductive skids or rollers on the energized powerline pair from a helicopter. Also, these support structures may inadvertently move into dangerous proximity with a grounded tower structure.
In an attempt to eliminate bulky, difficult to position, linemen support platforms, systems have been developed for lowering a lineman from a helicopter directly onto an energized powerline. U.S. Pat. No. 4,673,059 to Kurtgis discloses such a system. Not only is there some difficulty in accurately positioning a lineman at the end of an elongate, swinging cable adjacent to an energized powerline for a time sufficient to complete a safe interconnection with the powerline, but once on the powerline, the lineman, through inattention, may move too close to a grounded tower.
It is a primary object of the present invention to provide a novel and improved helicopter lineman""s ladder and method which permits a lineman to be directly landed from a helicopter onto a stable powerline support tower.
Another object of the present invention is to provide a novel and improved lineman""s ladder which is lightweight and which can be lowered from a helicopter and easily positioned between a tower cross arm and energized conductors by a single lineman.
Yet another object of the present invention is to provide a novel and improved helicopter lineman""s ladder which will deliver a lineman from a tower to energized powerlines and insure that the lineman is a safe distance from the tower before the lineman bonds to an energized powerline.
A still further object of the present invention is to provide a novel and improved helicopter lineman""s ladder which operates to prevent a lineman who has bonded to an energized powerline from inadvertently moving dangerously close to a grounded tower.
These and other objects of the present invention are achieved by providing a rigid ladder having one end which hooks to a powerline support bridge after the ladder is dropped by helicopter. The ladder includes a first section which extends out from the support bridge for the flashover distance to the tower or more, and a second section that extends downwardly from the outer end of the first section to a base which rests on the powerlines spaced from the support bridge. A weight on the base holds it against the powerlines, and the complete structure is formed of electrically nonconductive material. Using this structure, when a lineman hooks up to the powerline, he will be more than the flashover distance from the support bridge.